A multitubular reactor is used for a reaction in which a raw material is brought into contact with a solid catalyst loaded in the reactor. The multitubular reactor controls the reaction temperature by efficiently removing large heat of reaction generated from a reaction of vapor phase catalytic oxidation in which a substance to be oxidized is brought into contact with molecular oxygen in the presence of a solid catalyst. In general, the reactor is used when there is a need for preventing the deterioration of the catalyst facilitated by the exposure to excessively high temperatures of the reaction heat.
In such a multitubular reactor, a fluid for cooling (hereinafter, also referred to as a heat medium) is circulated outside a reaction tube assembly (i.e. on the side of a shell) to maintain the temperature necessary for the reaction, while heat exchange between a process fluid (in the case of reaction of vapor phase catalytic oxidation, a process gas) and the heat medium is simultaneously conducted as performed in heat exchangers widely used in chemical plants. This process prevents the catalyst in the tube from deteriorating owing to the excessive local temperature rise in catalyst layer (the formation of hot spots).
However, the heat of reaction from the reaction of vapor phase catalytic oxidation is so large as to cause the deterioration of the catalyst due to the frequent occurrence of hot spots and to cause a runaway reaction by exceeding the allowable temperature of the catalyst. This can result in problems such as inability to utilize the catalyst.
Numerous methods of restraining the formation of hot spots in a multitubular reactor used for a reaction of vapor phase catalytic oxidation have been proposed. In a method disclosed, for example, in JP 08-92147 A, the direction of flow of a heat medium within a reactor shell and the direction of flow of a raw material gas directed by the reactor are made parallel. In addition, the flow of the heat medium is meandered with baffles to move upward. Thereby, the temperature of the heat medium is rendered uniform with a 2–10° C. or less temperature difference from the inlet to the outlet. However, the method pays attention to only the temperature difference of heat medium. Thus, in an actual reactor having an uneven heat transfer coefficient therein, the method poses a disadvantage of generating hot spots in an area with a poor heat transfer coefficient.
JP 2000-93784 A has proposed a method of restraining the formation of hot spots in which the flows of reacted raw material gas and a heat medium are made downward parallel to prevent the gas accumulation containing no heat medium. It has further described a method of making only the catalyst around the entry of catalyst layer which is most easily deteriorated exchangeable by supplying raw material gas into the reactor via the upper portion thereof to pass downward through the catalyst layer of the reaction tube. However, the method focuses on the relationship of raw material gas flow with the heat medium. Thus, it has a disadvantage of insufficiently removing the heat of reaction to generate hot spots if the flow velocity of the heat medium and the heat transfer coefficient are low.
Alternatively, JP 2001-137689 A has proposed a method of restraining the formation of hot spots by defining how baffles, which change the direction of heat medium flow, and reaction tubes are placed. In the multitubular reactor, a heat medium for cooling the heat of reaction is circulated on the side of shell thereof. Owing to the existence of the reaction tube assembly and the baffles in a flow path on the side of shell, the heat medium flows separately into the reaction tube assembly, into a space between the baffles and the reaction tube assembly, and into a space between the baffles and the reactor body. However, the heat medium passing through the portion other than the reaction tube assembly is not useful for cooling the reaction tubes, and thus the amount of such a medium should be reduced as much as possible. Also, JP 2001-137689 A has a description relating to the flow rate of all heat media but has no description relating to a heat transfer coefficient. Therefore, the problems such as hot spots must have been alleviated by taking the heat transfer coefficient into consideration.
In a multitubular reactor, the heat of reaction which occurs within reaction tubes is removed by the circulation of a heat medium. Thus, if the heat of reaction is not effectively removed, hot spots are formed in a catalyst layer, resulting in the reduction in the yield of a desired product, the deterioration of catalytic activity, and the like.
The temperature distribution of the catalyst layer is determined by the balance between the amount of heat generation within the reaction tube and the amount of heat transfer to the heat medium. Accordingly, the approach to decrease the temperature in hot spots has been attempted in which the heat transfer coefficient on the side of the heat medium is increased by giving the larger flow rate of the heat medium. However, augmenting the flow rate of the heat medium more than necessary causes the increase in size of a circulation pump for the heat medium. Moreover, the larger power for driving the circulation pump for the heat medium is required, resulting in a problem in that an operation cost increases.